The present invention relates generally to electrical transformers. More specifically, it relates to high voltage, high current electrical transformers for use in welding power supplies, plasma cutters and induction heaters.
High frequency transformers operating at high voltages and high currents are commonly used in welding power supplies. The output stage of a welding power supply, for example, may include an electrical transformer to transform the high bus voltage of the welding power supply into a high current welding output. Transformer primary coil voltages on the order of 465 volts at 20 to 100 Khz and secondary coil currents on the order of 400 amps are typical. Welding power supply transformer coils (e.g., primary and secondary coils) are made from large diameter wires (3-14 gauge wire is typical) in order to handle the temperatures generated by these large voltages and currents.
Most of these transformers include a central bobbin having a coil winding window disposed about a central opening in the bobbin. The central opening is provided to receive one or more laminated or ferrite magnetic cores. Standard off-the-shelf magnetic cores are available in a wide variety of sizes and shapes, many of which have square or rectangular cross-sections. The coil windings typically also have rectangular or square cross sections wound close to the magnetic cores. This is because it is generally desirable to keep the coil windings close to the magnetic core to maximize the magnetic coupling between the magnetic core and the coil windings.
Having coil windings with rectangular or square cross sections can be problematic in welding applications however. This is because the large diameter wires used in welding power supply transformers have a tendency to deform or bulge at locations where the winding direction changes quickly (e.g., at the corners when wound around a bobbin having a square or rectangular cross section). This is especially true for Litz wire, a stranded woven type of wire used extensively in high frequency (e.g., 20 Khz to 100 khz) welding power supply transformers. The outer insulation that is placed over these large wires can also bulge and deform.
The width of the overall coil winding in the area of the deformations tends to be wider than the width of the remaining portion of the coil because of the bulging wires. As a result, the coil may not fit within the winding window of the bobbin in those areas. At the very least, extra manufacturing steps, typically manual, must be taken during the coil winding process to properly fit the deformed coil into the winding window in the vicinity of the bulges or deformations. It is desirable, therefore, to have a bobbin winding window cross section that does not have quick changes in winding direction. Preferably, the central opening in the bobbin will still accommodate standard size, readily available, magnetic cores having rectangular or square cross sections.
Another problem with using large diameter wires in welding power supply transformers is that the wire leads to and from these transformers tend to be less flexible than smaller wire leads. Extra space has typically been available inside of the welding power supply chassis around these transformers to allow the high voltage and high current transformer leads to be safely routed and connected to the rest of the welding power supply.
The current trend in designing welding power supplies, plasma cutters and induction heaters, however, is to make these devices smaller. One way to accomplish this is to pack the various power supply components closer together inside of the chassis. As a result, other power supply components are placed closer to the high voltage, high current transformers in these designs. Less room is thus provided to safely rout the leads from the transformer to the rest of the power supply.
It is desirable therefore to have a welding power supply transformer wherein the leads exit the transformer in a known and repeatable manner. Preferably, the transformer structures will have smooth edges and surfaces in the vicinity where the leads exit the transformer to prevent damage to the transformer leads.
Another problem with welding power supply transformers, especially welding power supply transformers operating at high frequencies, is leakage inductance. The presence of high leakage inductance in these transformers can cause several problems. A leaky output transformer can reduce the output power of the welding power supply. The primary and secondary coils in leaky transformers are more susceptible to overheating. Finally, the energy stored in the leakage inductance can be detrimental to transistor switching circuits in the welding power supply. Release of this stored energy can cause ringing, transistor failure and timing issues. Reducing or minimizing the leakage inductance in welding power supply transformers is therefore generally desirable.
Leakage inductance results from primary coil flux that does not link to the secondary coil. The amount of primary coil flux linked to the secondary coil is dependent on the physical orientation and location of the primary and secondary coils with respect to each other. Reducing or minimizing the mean distance between the turns of the primary coil and the turns of the secondary coil will typically reduce or minimize leakage inductance in a transformer. Reducing or minimizing the mean length of the turns in a coil will also typically reduce or minimize leakage inductance.
It is desirable, therefore, to reduce or minimize the mean distance between the turns of the primary coil and the turns of the secondary coil in welding power supply transformers. Preferably, the mean length of the turns in the coils of the transformer will also be reduced or minimized.
According to a first aspect of the invention, a welding-type power supply transformer includes a bobbin having elongated top and bottom surfaces and first and second substantially semi-circular end surfaces connecting the top surface with the bottom surface to form an elongated first coil winding surface having a central axis. A first coil is wound around the first coil winding surface of the bobbin. A second coil is magnetically coupled to the first coil.
In two embodiments, the transformer also includes an insulating shroud disposed between the first coil and the second coil. The insulating shroud includes elongated top and bottom surfaces and first and second substantially semi-circular end surfaces in one of the embodiments. The substantially semi-circular end surfaces connect the top surface with the bottom surface to form a second coil winding surface. The second coil is wound around the second coil winding surface in this embodiment. The second coil includes a plurality of second coil turns in another embodiment. The transformer includes a plurality of locating bosses in this embodiment disposed on the second coil winding surface to maintain each of the plurality of second coil turns in a desired location.
In the other embodiment, the insulating shroud includes a second coil winding surface and first and second insulating shroud sidewalls. The sidewalls are each disposed along opposite sides of the second coil winding surface. The second coil winding surface substantially conforms to the shape of the first coil in this embodiment and the second coil is wound around the second coil winding surface between the first and second insulating shroud sidewalls.
The bobbin includes a central opening disposed inside of the first coil winding surface in another embodiment. A magnetic core is disposed in the central opening. The magnetic core has a rectangular cross-section immediately adjacent one of the first or second substantially semi-circular end surfaces. In yet another embodiment, the second coil includes a plurality of second coil turns. A plurality of locating spacers are disposed to maintain a desired spacing between each of the plurality of second coil turns. The plurality of locating spacers are disposed such that there is at least one locating spacer between each second coil turn in one embodiment and such that there is at least one locating spacer on each side of each second coil turn in an alternative embodiment.
In another embodiment, the bobbin includes first and second bobbin sidewalls. Each sidewall is disposed along opposite sides of the first coil winding surface to form a winding window. The bobbin also includes first and second wire exits adjacent to and in open communication with the winding window. The first coil includes a first lead end exiting the winding window through the first wire exit and a second lead end exiting the winding window through the second wire exit. The first lead end and the second lead end exit the bobbin in a direction that is substantially perpendicular to the central axis in this embodiment.
The second coil is wound concentric to the first coil in one other embodiment. The transformer includes a cover disposed such that the first coil and the second coil are compressed between the first coil winding surface and the cover in this embodiment.
According to a second aspect of the invention, a welding-type power supply transformer includes a bobbin, a first wire exit, a first coil and a second coil. The second coil is magnetically coupled to the first coil. The bobbin has a central axis and a first winding window located about the central axis. The first winding window includes a first coil winding surface and first and second bobbin sidewalls each located on opposite sides of the first coil winding surface. The first wire exit is in open communication with the first winding window. The first coil is wound around the first coil winding surface and includes a first lead end. The first lead end exits the first winding window through the wire exit such that the first lead end exits the bobbin in a direction that is substantially perpendicular to the central axis.
The transformer includes a second wire exit in open communication with the first winding window in another embodiment. The first coil includes a second lead end exiting the first winding window through the second wire exit such that the second lead end exits the bobbin in a direction that is substantially perpendicular to the central axis in this embodiment. Each of the wire exits is disposed adjacent to the first winding window in another embodiment.
In one embodiment, each wire exit includes an outside wall and a rear wall. The rear wall is connected to the bobbin sidewall along a first edge and is connected to the outside wall along a second edge. The first and second edges are radiused on the inside of the wire exits in this embodiment.
In another embodiment, the second coil includes a plurality of second coil turns. A plurality of locating spacers are disposed to maintain a desired spacing between each of the plurality of second coil turns. The plurality of locating spacers are disposed such that there is at least one locating spacer between each second coil turn in one embodiment. The plurality of locating spacers are disposed such that there is at least one locating spacer on each side of each of the plurality of second coil turns in an alternative embodiment.
The second coil is wound concentric to the first coil in one embodiment. The transformer includes a cover disposed such that the first coil and the second coil are compressed between the first coil winding surface and the cover in this embodiment.
According to a third aspect of the invention, a welding-type power supply transformer includes a bobbin, a first coil, a second coil and a cover. The bobbin has a first coil winding surface. The first coil is wound around the first coil winding surface. The second coil is wound concentric to the first coil. The first coil and the second coil are compressed between the first coil winding surface and the cover.
The transformer further includes a plurality of compression bosses in one embodiment. Each of the plurality of compression bosses contacts one of the first or second coils to compress the first coil and the second coil between the first coil winding surface and the cover in this embodiment. At least one of the plurality of compression bosses is located on the cover in one embodiment and at least one of the plurality of compression bosses is located on the first coil winding surface in another embodiment.
The second coil is disposed on the outside of the first coil and an insulating shroud is disposed between the first coil and the second coil in other embodiments. The second coil includes a plurality of second coil turns in one other embodiment. The plurality of locating spacers are disposed to maintain a desired spacing between each of the plurality of second coil turns in this embodiment.
According to a fourth aspect of the invention, a welding-type power supply transformer includes a first coil and a second coil magnetically coupled to the first coil. The second coil includes a plurality of second coil turns. A plurality of locating spacers are disposed to maintain a desired spacing between each of the plurality of second coil turns.
Each of the plurality of locating spacers is disposed such that there is one locating spacer between each second coil turn in one embodiment. The plurality of locating spacers are disposed such that there is one locating spacer on each side of each of the plurality of second coil turns in another embodiment.
According to a fifth aspect of the invention, a method of reducing the leakage inductance in a welding-type power supply transformer includes providing a first coil. A second coil is wound concentric to the first coil. The first coil and the second coil are compressed together to reduce the leakage inductance between the first coil and the second coil to a desired value.
Other principal features and advantages of the invention will become apparent to those skilled in the art upon review of the following drawings, the detailed description and the appended claims.